Liquid level indicator



May 25, 1954 A. wExLER 2,679,642

` LIQUID LEVEL INDICATOR WITNESSES: INVENTOR M Aaron Wexler.

ATTORNEY Filed June 23, 1951 3 Sheets-Sheet 2 WITNESSES: INVENTOR Auron Wexler.

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ATTORNEY May 25, 1954 A. wExLER LIQUID LEVEL INDICATOR 3 Sheets-Sheet 3 Filed June' 23, 1951 wlsfl- R I S6 l 2 l R RC C m. C. F 3 7v I. 2 L 8, o\ v 8 s N R 5 7 g 8, .I a 68 F O b 97 V 8 n 7 9 4 9 .2o .3o Current .-Amps.

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af/Mm ATTORNEY Patented May 25, 1954 LIQUID LEVEL INDICATOR Aaron Wexler, Pittsburgh, Pa., assignor to Westinghouse Electric Corporation,

East Pittsburgh,

Pa., a corporation of Pennsylvania Application June 23, 1951, Serial No. 233,195

3 Claims.

This invention relates generally to electric gauges and more in particular to gauges of the type which are useful in indicating the level of liquids in containers.

More in particular this invention relates to an electric gauge for liquid level measurement which embodies a wire filament of a material capable of carrying a predetermined electric current and adapted for immersion, at least in part, in a liquid the level of which is to be measured, the Wire filament having a suiciently large temperature coefficient of resistivity that the resistance of the wire in the liquid is diierent from that in the atmosphere or vapor above the liquid for the given current. The resulting voltage diierence therefore forms a basis for ascertaining the level of the liquid.

In connection with the determination of liquid level of low boiling point liquids, there are four general methods which have been utilized. The most direct method involves the weighing of the container. This method is clearly not applicable when the container is rigidly mounted as may frequently be the case nor is it a practical expedient when the ratio of the weight of the container to the weight of the liquid is large as is usually the case when such liquids as hydrogen and helium. Further, when the internal geometry of the container is not simple, an indication of the level of the liquid is frequently more useful than one of the total contents.

Another method is based on the measurement of the hydrostatic head' exerted by the liquid. This usually requires the use of draft gauges which are easily upset by pressure surges within the container, resulting in transient pressure differences many times greater than that exerted by the liquid.

A third method is based on the fact that the temperature of the vapor above the liquid may be higher than that of the liquid. Gauges based on this principle, however, are not usually capable of reproducing indications because normally the temperature difference is small and variable for a variety of reasons.

The fourth method is based on the fact that the heat transfer from a body to a boiling liquid is better than that between a body and the vapor above the liquid. Advantage is taken of this principle in electric gauge arrangements in which there are a number of thermocouples with heaters mounted at various levels in a container. The output voltage of a given thermocouple is signicantly different, depending upon whether it is immersed in the liquid or is in the vapor.

This type of gauge Works satisfactorily but is complex in structure requiring relatively sensitive indicating instruments. A further disadvantage of the thermooouple system is thatI the surface of the unit to be heated' is relatively large and, therefore, relatively large amounts of power are required for increasing its temperature sufriciently to give a positive indication.

One object of this invention is to provide a liquid level gauge which is simple in its elements and positive in operation.

A further object of this invention is to provide an electric gauge embodying an element to be at least partly submerged in the liquid, the level of which is to be measured, in which advantage is taken of the fact that the heat transfer between that portion of the element immersed in the liquid and the liquid is better than the heat transfer between the element and.' the vapor above the liquid.

More specifically stated, it is an object of this invention to provide an electric gauge embodying a. strand of metal having a temperature coefficient of resistivity, which strand of metal is adapted to be immersed at least in part in the liquid, the level of which is to be measured and to have a current passed therethrough, whereby the voltage variation of said strand may be utilized as an indication of the level of the liquid.

The foregoing statements are merely illustrative of various aims and objects of this invention. Other objects and advantages will become apparent from a study of the following specification when considered in conjunction with the accompanying drawings, in which:

Fig. 1 is a view fragmentarily in section of one embodiment of this invention;

Fig. 2 is a sectional view illustrating the application of a second embodiment of this invention;

Fig. 3 is an enlarged fragmentary sectional view of the secondA embodiment of this invention;

Fig. 4 is a detail of the sensitive element of Fig. 3 drawn to an enlarged scale;

Figs. 5 and 6 are diagrammatic illustrations of circuits applicable to the second embodiments illustrated in Fig. 3 and Figs. 7, 8 and 9 graphically illustrate electrical characteristics of two types of sensitive elements embodied in this invention.

The assembly illustrated in Fig. 1 includes a container I which is partly filled with a liquid 2. This container is sealed by means of a cover 4. The use of the cover 4 is not essential to this Itremitiesof the fsplit tubular sections.

vseal -glass-*to-metal seal il 8 Iaddition to providing a from the split housing contact therewithfand at its outer electrodes -20 protrudes.

trudes through the `sealed fthereto 'to form a .liquid-'tight joint by invention, but serves in part to support the upper extremity of a liquid level gauge assembly G, which is supported within the container. The use of the cover will depend largely upon whether or not contamination of the fluid can be tolerated and upon whether or not it is desired to insulate as much as possible the contents of the container from room temperature. In the -application of this invention to low boiling point liquids, it will be appreciated that more elaborate precautions than those illustrated in Fig. 1 (see Fig. 3) will ordinarily be taken to minimize heat transfer between the room atmosphere and the liquid within the container. The lb'ottom portion `of the liquid level gauge assembly'extends through the bottom of the container.

The liquid level gauge is comprised' of a Ksplit tubular section 8 which is longitudinally split in diametrically opposite positions. One longitudinal opening is designated l0 and the other is 'designated Ia. The-split' tubular section is secured at each -of its extremities,I .as by soldering, to Aform a liquid-tight\=joint, to short Ysleeves l2 `which are outwardly flanged 'adjacent the 'exsembly of split tubular sections forms aihousing for the vsensitive element :i4 of the gauge, 'which element is a fine-strand oflmetal. Such a'strand 'of-metal,"for example, may bea ne wireof .pure

platinum having adiameter of about 1/1000 oi an inch.

lSleeves :I6 are .concentrically ydisposed within the'short sleeves .12- at leach 'endof .the `split Ahousing and 'are :secured .therein by a vglass-to-rnetal t8 at each end 'of 'the housing. The

secure and lliquid-tight support yfor the sleeves I'B also'serves tolele'ctrically 'insulate the sleeves 8. 'Terminals `or electrodes 2l! are respectively slidably iitted through the sleeves I6 in each end of the-'housing and protrude from theinn'erfends of the sleeves into 'the housing the `'strand I4 y'being lspot welded to the tips of the terminals to havegoo'd electrical to ibesupported thereby.

Each sleeveuis -tted with a tapped blocki22 extremity through which each nf the Set screws 224 :threaded `through lthe Yside Tof eachof the tapped blocks Iengage the terminals ;20 and secure these terminals in their proper positions. .The bottom end 'of the-liquid levelrg'auge assembly which .pro-

bottom foi 'the `container .I `is means of a glass-tometalIseal228. This ,seal :is

-suiiciently strong -t'o support the'gauge assembly in the container, in addition -to forming Athe fluidtight joint previously men'ioned Leakage of 'iluid through sleeve vIi 'around electrode'20at the bottom of the container may be obviated "by soldering the'bottom electrode iniits sleeve.

The strand' |`4 as previously noted "is of'a vpure metal. Most .pure metals havepositive temperature coefficients 'of `resistivity. 'The 'resistance of the Wire or the voltage drop across it "for a given current will be markedly .greater when it is in the vapor than when itis in the liquid. Furthermore, in the case of a 'fine Wire, along which thermal conduction may be .neglected in comparison with the total .power generated, the

.resistance or lvoltagedrop is essentially a linear function of the `liquid level. In'vone vpractical applicatiomlane wire of .l/wgoof-an inch diam- This asi:eter.platinum was employed. However, afmetal '75 earlier-noted, is -fessentia1lya linear 4 such as tungsten and other pure metals may also be employed for this purpose.

In the application of this embodiment of the present invention to the measurement of the level oi low boiling point liquids, the metal selected is preferably one which does not become superconductive at temperatures at which the gauge will operate. Other-Wise, there lwill be no possibility of 'producing Joule `heat on which the operation of the gauge depends.

Provision for filling and emptying the containeriseaiorded by the tube 25 secured by the bushing 25 in tube 28 which, in turn, is secured inthe cover. Tube125 is of U-shaped congura- 1tion 'having an'extended section terminating ad- 'jacent the 'bottom 'of the container. For filling the container, the liquid supply is applied to the external end of the tube 25 and the container is vented through a tube 21 mounted in the cover 4. For emptying the container, a supply of gas :under suitable pressure is 'applied to tube 21 :forcing the :liquid contents through the tube '25. If liquid l2 is-a `low boiling point liquid, the source or gas pressure is usually theigaseous form of the liquid tofminimize liquid contamination.

Various types-'of electric circuits may be `uti- .;lized :in 'connection 'with 4the gauge herein rillusitrated. Such circuits, depending upon the degree -of accuracy desired, maybe `arranged to compensate various unwanted variables resulting from external or internal conditions. :For ex- `ample, ambient temperature variations, Yan 'exter- -nal condition, may disturb the calibration. Y.The sensitivity of vthe gauge 'assembly '6 to ambient .temperature is minimized to .a large extent .by

provision ofthe metallic housing. In a practical embodiment of this invention, this housing 'is made Vof -copper and substantially guarantees that the :ambient temperature as far as the :strands-of wire 14 `:is concerned, remains constant :irrespective of the liquid flevel height.

A Asimple circuit for energizing the'electric fgaugeisillustratedin Fig. 1 and includes-a source `of electrical energy S herein 4designated `as a -source of alternating current. However, .it listo *fbeappreciated lthat .direct fcurrent =may also be remployed. The -source is connected iin series .with a current limiting resistor R across -the 'terminals 20 ofthe liquidrlevel gauge. The value rif-resistor vR vdepends ron the extent to which it is to be current limiting. In most cases itfis iof such a =value as rto 'maintain the current through -thegaug'e Aelement constant, irrespective of fits resistance.

Upon -the application of Voltage to the "ter- .minals 20 lby the source, `a -current is `caused to row through the series circuit includingthe 1resistance R and the strand o wire 14. This -current is selected of isui'cient magnitude to cause the wire 'I4 to become zheated. It will'lbe 'appreciated that the portion'of the vire `which Y-isout of the 'liquid 2'Will 'be'.appreciablywarmer .than that `which Lis :immersed in the .liquid 2-and,

further, thatlas the liquid level falls due to yre- Ythepreviouslyconsidered'case in which the Il l,

:level 'was higher. yThis increase =in 'voltag cunard -ment I4.

the container 42 illustrated in Fig. 2.

the liquid level and as a consequence, the change in voltage across the gauge element is an indication of the variation in liquid level and may be conveniently measured by means of a volt meter such as V connected across terminals 38 and having a suitably calibrated scale. The meter circuit conductors to terminals 26 are preferably separate from the energizing circuit conductors, as shown. This expedient minimizes or eliminates instrument circuit response to voltages other than those across the strand or la- For example, this expedient eliminates from the instrument circuit the voltage drop along the energizing conductors due to the flow of energizing current therein.

It should be noted at this point that other types of circuits may be employed in accordance with conventional practice in the metering art'. For example, the electric gauge may be connected as one leg of a, bridge circuit, in which case the unbalance voltage across the bridge circuit is an indication of liquid level and in connection with the use of a bridge circuit, various conventional expedients may be utilized to minimize effects of unwanted variables in the metering system. However, for most applications the simple basic circuit herein illustrated will probably be sufficient. Y

In actual tests which have been made with an electric gauge according to this invention and with a platinum wire of 1/1000 of an inch diameter and 14 centimeters in length, it was found that a current of .17 ampere was a good operating current for liquid nitrogen, since the resistance of the wire Was iive times as large in the vapor as in the liquid. For these conditions, the linearity of the gauge is good and the voltage swing across the gauge at constant current varies between 1.2 volts and 5.3 volts. Additionally, the gauge indications of liquid level are entirely reproducible over the complete range. For liquid helium, .425 ampere is known to be a good operating current.

For a given temperature rise of the wire in the vapor, the following relationships give the dependence of current voltage and power on the radius of the wire:

In the second embodiment of this invention,

principles analogous to those utilized in the previously described embodiment are employed. The physical arrangement, however, is somewhat different. The arrangement being in the form of a dip stick, generally designated lil which is adapted for insertion into a container such as The dip stick is provided with a head 44 of electrical insulating material within which the ne strand of Wire is supported. As will be seen by rer"- erence to Fig. 2, the end of the head 44 is partly immersed in the liquid, generally designated 46.

The container of the type illustrated in Fig. 2 is frequently employed in storing liquid helium. Such a container includes an outer vacuum jacket assembly 48 within which is positioned a vacuum jacket ilask 58 in which the liquid helium is stored. The container section 48 is lled with a low boiling point liquid such as liquid nitrogen to a level indicated by the liquid l level line 52 which extends an appreciable distance up the neck of the flask above the surface of the liquid helium 46 within the flask. The outer container shell 48 is closed over by means .of a cover 54 which is provided with a hole therethrough (not shown) through which the neck be positioned beneath the level of the liquid helium in the flask. This thin-walled tube is pref.- erably of a material of low thermal conductivity.

To the bottom end of the thin-walled tube 58, the head 44 is secured. This head is comprised of three separate sections of electrical insulating material. One section 60, which is the main body portion, is provided with a pair of reduced diameter sections 62 and 68. Section 62 is press-fitted into the lower end of the thin-walled tube 58 until it abuts the shoulder formed by the larger diameter section 68. This main body portion 68 is essentially circular in cross-section and is provided with a cylindrical cavity I0 which opens through the bottom thereof. Diametrically disposed openings l2 extend through the side walls of body portion 60 at points adjacent the uppermost end of the cavity as viewed. The open end of this cavity is closed by means of a cap 'I4 which is press-fitted therein and which is provided with a pair of drain holes 'I6 in order to drain liquid. A bullet-shaped cap 59 having an outer diameter corresponding to that of body portion 60 and shaped internally to slidably t over thinwalled tube 58 and seat upon the shoulder formed by section 68, is secured in position against the mentioned shoulder by means of a pin 59a secured in body portion 60 and projecting into the inner wall of cap 59. This cap assures smooth withdrawal of the gauge head from the flask in which it is inserted through the neck of such flask.

Two short sections of brass capillary tubing 18a and '18h are fitted into the the main body portion 60 and extend longitudinally thereof through the main body portion on one end into the cavity 10 and on the other end, through the reduced diameter section 62 into the thinwalled tube 58. The upper ends of tubes '18a and 18h are connected to a plurality of conductors 19, 80, 8| and 82 which extend through the full length of the thin-walled tube 58. Although the details of these connections are not shown, it is to be understood that such connections are conventional, the ends of the conductors 19 through 82 being soldered to the tubes or terminals 18a and 18h. Each of the terminals has two of the conductors connected thereto. Thus, the conductors 8| and 82 eX- tend down through the thin-walled tube 58 and connect to the terminal on the left as viewed while the conductors 'I9 and 80 connect to the terminals 'l8b on the right as viewed (also see Fig. 5).

The upper ends of the conductors 19 through 82 are each connected into separate terminals of the group of terminals 84 of a standard plug 85. This plug is of a type in 'which the terminals 84 are sealed in glass internally of the plug, the plug casing being formed of a suitable plastic material. The plug assembly is such that it forms a gas-tight unit and is mounted in a plug housing 86, which is substantially of :acid to dissolve .oi the copper.

:envases :the thin-twalled tube :58 :.an'd iis fsealed theretoeas iby :soldering to -form 1a gas-tight joint. 'flug 85 .is .tted into 'an enlzztrgler'l'k portion 617 ioffthe plug fhousing 86 .adjacent-:the upper iena thereof .and seats .upon ia gasket 8,8 which, in turn, :is

.seatedzupon ,aashoulder f90 aformed :at`the base "of .the enlargediportion. :Plug r85 -iis `:secured .infithe plug housing against the gasket by means of a .plug retainer ring '92 which .is internally threaded :and threads over'zt-he fenlarged portion 86h of the housing. This retainer ring is Tpro- `--vided with an :inwardly anged upper #extremity Vwhich 'rengagesV the upper .surface of @plug .85 'to z apply :the seating and `securing fpressure.

In this :embodiment `.of "the invention, `fthe Ysensitive, element 194 may be 'a 1.1/8 inch :long section of'.00005 in. dia. platinum zwireisolderedto y'.the1ends .of la vtwo-.wire press 595 of :the ztype .used

inithe manufacture lof t-lashlight bulbs. This press .comprises two wires fSB -iwhich -are fused into :arceramic 'headl. "The-.ends of :the wires "86 on Vwhich the l:Wolfram coil iis normally mounted in the manufacture-tof the :;bulb, vare 'the' process :of manufacture of :these :small:bu1bs, M'the wires .'96 :are .coated ywith :red phosphorus .whichfeacts a fgetter when rheated.

In 'manufacturing the `temperature sensitive aunit, :the first step'ds to abend vthe ends ofnthe wires 96 to -thewshape indicated. v'Il-hisxnayizhe .accomplished 'with the-'help :of a l/ inchsquare .of metal so as 'ato gprovide -for -,a wuniiorm v:beyond-the ceramic head. Thefrst leinchioi thefendsof wires^;9,6.are.,treated .with a 50% v,aqueousfsolution:of warm :nitric This :is rdone 1 to minimize bubble-formation vlater-iin the process .when `,bubbles-zmight 'break v.the Nery :ne `wire element. .The wirefpress -is :then ctreatediwith dilute hydrochloric .facid to dissolve the ,phosphorus. The .00005 in. dia.?p1atmum=wire,awhich .is manufactured=with alprotective coating 4offsilver ther-cover, is :mounted :b y vtafking` two :turns of 4this sensitive 4Wire element aboutfonerofrithe hooks. The wireisthenipulled nrrnlyto'za'zposivtion .adjacent fthe rrother fhook and two turns :are -wrapped upon the second hook, essentially ;as ishown. V'fheiwire 1is fthenfcut fromthe esupply reel and :the :hooks 918 alter-closed with aipair'of ,-pliers to .form a good :mechanical I .connectionrat both tends.

The `sensitive wire element is then treated '.withfa #50% zaqueous solution rc1 :warm nitric f acid torremoye ...the -silverjaciiet Thesrne .wire `is carefully :rinsed :with water to v:remove yexcess "acid, -.Positive :electrical :connections .are

made atieitherl-end by;carefully..'applying-:a-small lamount zof s60-40 "solder with tacid `iiux. fCare is taken toevaporate-.allzthe flux inithis operation. About `/ifelements canibe :made .from-oner-.foot-.of .thene wire with '.this process In .this -fembodiment .fot 'the invention, ythe wires zareinserted; into the endsiof:theztubesforiter-` i f minals 18a. and 58h which project intorthe'cavity .'12 asfshown finfzig. .3. Thereafter, mires 96 :and Ytubular :terminals 'LBand lbaare.securely l.joined as .by soldering.

The :circuit which =is `herein :employed-:is similar .to that :illustrated in Fig. 1. `Thiscircuit vis:illustrated-in Fig'. It'alsofappearsdn Fig. LlneFig. 5, the r-iterminals .=.18a and ilbeare :repre- .sentedrbyztneisimilarlyiidentinedgjunction;points -dnzthe circuit. Airesistance :or impedance :device .ned .gas within the. container.

designated .R :is connectedzin-series .the icon- Aductor 19 to function 4as .fa current limiter, las

.Here iagain, direct cunent may b e employed'iif desired. 'With the .arrangement illustrated, it

`will be y.appreciated that ione set Iof conductors l nod32 are .provided to conduct .the `energizing `current :to .thecsensiti-ve element :94 and that a .separate set lof fconductors .8D and .'l isprovided for taking `voltage readings :of the voltage across the sensitive ielement. `The Noltmeterzdesignated V is connected across the ends .of :con- Aductors 480 and .iB-l. The .objectzin-providing these separate conductors is iagain A.tominin'1ize:the possibility ci reading .Voltage drops due `to the resistance of theconductors lcarrying the .zcurrent to and away from ,the element .94. Thus the measured voltage is Aprimarily v.indicative :of the 'Voltage across the sensitive element Sil.

The room temperature resistance :of Va sensiltive element suchas S4 is about .120 ohms. The

resistance is about 30=ohmsfat the nitrogen boiling point and about V9 ohms at the. helium boiling point. It is found ,that a ,good operating current for the liquid nitrogen-is .011 ramperezgiving a fourfold resistance pincrease when :the awire ,is in the vapor. A good operating current 'for liquid helium is .020 ampere giving a threefoldresistance increase .when ,thewireisnthe vaporiabove the liquid helium. .It 'is tto bennoted .that .the power `dissipated in .the liquid .isivery 4low,beng of the order o rabout .004'watt.

The metering .and energizing fconnections, it will .be appreciated, :are vmaclelthrough .-thefgterminals of the plugrii '.by .the use of afsocket, not shown, connected ato a meter `:and to asuitable supply of energizing current. The socketleads should be sufficiently long and flexible yto iprovide ready manipulation of `thisliquid levelmeasuring device. .In-practice, voncethe circuit `con- --nections areestablished this .device may :be lowered into a container of the type-*such :as `illustrated inFig.'2. .Upondnsertion into a fiaskisuch as 50, the .temperature-and the resistance'ofithe `,sensitive element vShri .drop .very `rapidly :until Vthe .change in resistance will be .reected externally as a sudden .movement o1 the pointer -of the .instrument V. This sudden ymovement of :the

`pointer then establishesthe depth. at which con- -tact with the liquidis made. If desired, thethin- Walled tube 58 mai,7 .be suitably .calibrated .externally to indicate theactual-.depth of the lque- A device suchas this may also be .used asiapermanent addition .to a liqueedgascontainer ofthe type shown.

This device may ybeslidably mounted in the containeratsome convenientpoint-so .that it always remains withthe `container-andnnay be utilized at any time to indicate the depth of theliqueed gas therein by the simple'expedient of connecting the device lto vthe energizing and'metering circuit through theplugzand `socket connections described.

The sensitive elementsxmayalso be usedinanother-way. A number of themimay mounted .at convenient Lpoints where :the presence or .fab-

sence f ofliquid f could Jae determined. .Theyzzmay scribed have been found to produce robust indications of the presence or absence of the liquid at a particular point1 it has been found that gauge elements of Wires having low residual resistance ratios produce an even better response to the changing conditions between immersion in the vapor and immersion in the liquid, particularly liquid helium. Referring to Fig. '7, experiments which have been conducted with unannealed .0004 diameter platinum wire (identified as type A in Fig. '7) made by drawing heavier wire through dies, indicates that the resistance is proportional to the absolute temperature over a wide temperature range. This is wire similar to the type hereinabove generally considered, for example, in connection with Fig. 1. Thf: drawn wire for the purposes of this discussion, identied as type A. In Fig. 7, the curve marked type A depicts the resistance-temperature relation for the Wire. At very low temperatures, the resistance becomes independent of temperature. This temperature independent minimum value (Rf/Ro" C., from the curve, is equal to .112 in this case) is a measure of the total (chemical and physical) impurity content of the metal. The term residual resistance ratio is given to this value. For an ideally pure metal, the residual resistance near absolute zero `is Zero. In the case of the type of wire presently being discussed, the impurities are mainly chemical, for the residual resistance is decreased by only 14% by vacuum annealing. Fig. 8 illustrates the resistance-current characteristic of the type A Wire used as a liquid helium gauge. From the curves it will be noted that at about 0.214 amp. the wire resistance in the vapor is about 4.3 ohms and in the liquid is about 0.44 ohm giving a maximum ratio of wire resistance in the vapor to wire resistance in the liquid of about 10, which diminishes rapidly at current values above and below y this level.

A .0004 diameter wire made by a particular process has characteristics which may be preferred for this application. This wire is annealed at 525 C. for three hours under high vacuum conditions. With such a wire, the residual resistance ratio is .0022 which is less than that for the drawn wire hereinabove considered by a factor of 50 and is identified as type B in Figs. 'l and 9. As shown in Fig. 9, when the annealed wire is immersed in liquid helium at a temperature in the vicinity of its boiling point, the resistance of the wire remains substantially constant at the residual resistance level, until the current lpassing through it rises to .310 ampere. When the current rises in nitesimally above this value, the resistance skyrockets. If the circuit is not such as to make the resistance of the gauge element current limiting, the platinum wire would fuse. The resistance R provided in the circuit illustrated in Fig. 5 is selected of such a size to introduce the necessary current limiting feature. When the wire is in the vapor in equilibrium with the liquid, the resistance remains virtually constant until the current rises above .215 ampere. Here again the resistance rises 'by a huge factor.

A suitable value for the operating current of such a gauge is of the order of .250 ampere. If the gauge element is immersed in liquid helium, then the current through it can be adjusted to .250 ampere with only a negligible voltage drop (about .004 volt) appearing across the element.

At the moment the element is raised above the liquid, the voltage drop rises to about 2 volts. At this time, in the circuit of Fig. 5, the switch SI is opened and the unit is again ready to give a liquid level indication. For the purpose of operating convenience, as shown in Fig. 6, the operating coil Cl of a sensitive relay CR may be connected across the gauge, the contacts CRI opening the energizing circuit and the contacts CRZ .being utilized to ring a bell or other audible device when the gauge element is in the vapor. A holding circuit including contacts CR3 and the holding coil C2 of the relay in series therewith, across the source, serves to lock the relay CR out when the gauge is in the vapor. Any other suitable arrangement such as a lockout relay may be employed for this purpose.

All of the other liquid level indicators described hereinbefore behave reversibly in the sense that for a given current the .resistance depends only on whether the sensitive element is in the liquid or the vapor and not on its previous history. In the use of the very sensitive elements having low residual resistivities, cognizance must be taken of the irreversibility shown in Fig. 9. Thus the curves show that if such an element is immersed in liquid helium the resistance will remain negligibly small until the current exceeds the Value at about 0.310 ampere. The resistance then skyrockets to a value which depends on the extent to which the greatly increased resistance of the element limits the current. It is found experimentally that if when the element is on curve CD it is lowered into the liquid it will return to curve OA only when its resistance is less than a critical value of the order of 6.0 ohms. Since it can always be returned to curve OA by reducing to zero the current passing through it, in the use of such elements the circuit is so arranged as to be opened automatically when a voltage rise appears across the element. Thus only if the gauge is in the liquid is a negligible voltage signal obtained.

From the foregoing, it may be construed that elements having the smallest residual resistivities will give the most robust indications of liquid level particularly for liquid helium and will also dissipate smaller amounts of power in the liquid. This is an important consideration in` view of the low latent heat of vaporization of liquefied gases such as helium.

Although pure metals have been suggested for the temperature sensitive elements, other substances may be used. Electrical conducting materials having an appreciable temperature coefficient of resistance, such as certain semi-conductors, offer possibilities, especially at higher temperatures than those encountered with liqueed gases.

For higher boiling point liquids such as oxygen or nitrogen, it has been found that annealing exerts only a minor inuence on the temperature coefhcient of resistance and hence on the behavior of the element as a level gauge for such liquids.

Although this invention in certain of its aspects is directed primarily to principles and apparatus applicable in the measurement of the level of low boiling point liquids in containers, it will be appreciated that, in its more general aspects, this invention may be practiced in connection with liquids other than those having low boiling points and, in fact, with adequate control of the energizing current may be applied in measuring or ascertaining levels of volatile liquids. Accordingly, itis intended thattheforegoing disclosure and the showings made-in the drawings shall, be considered only as'illustrative or` theprinciples of this invention and not construed in a. limiting sense..

I` claim as my` invention:

l. In a device adapted; for insertion; into a container for determining the depth of a liquefied gas inthe container, the combination of, a

-hollow tube ofv a material ofV low thermal conductivity, a head of; electrical insulating material mounted onqone endof said tube in substantially gas tight relation'therewith to seal said one end of said tubesaid, headhaving an open cavity therein, a pair of terminals mounted `in .said head withfthe endsthereof projecting into said cavity and into said tube, wirelament means` con,- `nectedacross.saidterminals .in said cavity, a plug housing` mountedonthe other endof saidltube in gas-tight relation therewithfto seal said other end. said plughousing having aV shoulder therein, a gasket seated on=said.shou1der,.an. electrical plug having apluraiity ofy terminalsmounted and sealed therein, saidwplug` being :seated on said gasket, means securing. ,said plug `in said housing on said gasket, and electrical conductors` con.- nected between thevterrninals in said plug and the terminals in saidl head.

2. Apparatus asset forthin` claim; 1, in which each terminaLin said head has two conductors connected thereto, the other-ends of which conductors areconnected toseparate terrnnalsin saidV plug.

3. In a device adapted for insertion .into acontainer for determiningthe depth of` aliqueed gas inthe container, the'combinationof. a: hollow tube of a material of lowI thermal.` conductivity, a head ofelectricalinsulating material mounted on one-end'ofisaidztube in substantially gastight relation therewith; to seall said one.d endV of4v said tube, said head having an open cavity therein, a pair, of terminals mountediin saidfhead withthe ends thereof projecting into said cavity'andlinto said tube, wire yfilament meansconnected` across said terminals in said cavity, a plug housing mounted on. the otherA end of said. tube in, gastight relation therewith to seal. saidk other' end, said plugl being seated on said gasket, means securing. said pluginsaid housing'onsaid gasket, a rst pair of conductors connected on one end to` separate terminais in said plugA and on. the other end to separate terminals in said heada second pair of conductors connected on oneend to separate terminals in said plug -differentfrom theterminals to which said rst pair ofconductors is connected and in the other end to separate terminals ofI said head, circuit connectionsincluding impedance means connected with the terminals of said plug to which said first pair; of conductors is connected for applying electrical energy thereto to energizer said wire filament, and an indicating instrument connected to the terminals of. saidy plug to which said secondpar of conductors is connected, said head being adapted for insertioninto a` container ofliqueed gas to engage said filament with the liqueed gas.

References Citedlin the ille of this patent UNITED STATES PATENTS Number Name Date 2,050,128( Schlumberger Aug. 4, 1936 2,195,019 Bloornheart Mar. 29, 1940 2,361,872 Rodgers Oct. 31, 1944 2,398,375 Heenam Apr. 26,' 1946 2,524,933 Silverman Oct. 10, 1950 2,537,246 Thoresen Jan; 9, 1951 FOREIGN PATENTS Number Country Date 622,247 France Feb. 2l, 1927 513,427 Great Britain Oct. 12, 1939 CTI-IER' REFERENCES "Methods of. Measuring Temperature, a book by Ezer Griths pages 12-13, published by Charles Griiiin 42 Drury Lane London, England (1947). 

